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ATM

Computer Networks and Internets, Comer

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Once you have a path• Scheme 1: once found, the path does not change,

and channels within that path are dedicated to the particular communication exchange, data (can be) continuous – Circuit switching

• Scheme 2: once found, the path does not change, but channels are shared, data broken into packet– Connection-oriented packet switching

• Scheme 3: paths may change, channels are shared, data broken into packets– Connectionless packet switching

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ATM

• One example of Scheme 2 on the previous slide is ATM.

• ATM: Asynchronous Transfer Mode • “a … high-speed, full-duplex, connection-

oriented, fixed-length cell-switching scheme that is suitable for data as well as digitized voice and video.” (Computer Dictionary)

• ATM grew out of the ISDN standards and is part of B-ISDN (broadband-ISDN).

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Trying to be all things• ATM had very ambitious goals. It was designed to

handle voice and video as well as data– The voice/video put strong constraints on the speed and

steadiness of the transmission rate. – The connection-oriented and small, fixed cell size

decisions are both motivated by the voice/video constraints.

• It was designed to be both a LAN and a WAN technology.– The emergence of faster Ethernet technologies is pushing

ATM out of the LAN market but it remains a viable WAN technology.

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Connectionless vs. Connection-Oriented

• A “connection” is not only setting up a physical and/or logical path but also an agreement between the sender and receiver to communicate. – Connectionless means that one can begin to transmit

data without establishing a “connection”• e.g. sending (snail) mail or IP (Internet Protocol)

– Connection-oriented requires a connection• e.g. calling on the phone or TCP (Transmission Control

Protocol)

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Connection

• Connections used to be made by physically plugging wires into a switchboard.

• Early computer programming was done in a similar fashion.

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ENIAC

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Virtual circuit

• Analogous to the way programs moved from hardwiring to RAM (and some ROM), establishing communication channels also became more software based.

• The software-based connection from source to destination is known as a “virtual circuit.”– There must be a physical connection. Actually there are

many possible physical connections and the software selects the desired path.

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PVC and SVC• Somewhat analogous to programming in which we

have ROM (for long-term memory) and RAM (for short-term memory), in virtual circuits we have permanent virtual circuits (PVC) and switched virtual circuits (SVC). – A PVC has to be “configured” (a somewhat slow

process) by the company providing the connection. It is permanently available (nailed down) to the user. Analogous to ROM.

– An SVC is set up more quickly and exists only during a “session.” Analogous to RAM.

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Establishing an connection

• In the connection-oriented approach, transmission is requested and accepted.

• Recall the DB9 connector (from homework), in addition to the Receive Data, Transmit Data, there were pins such as Carrier Detect, Request to Send, Clear to Send, etc. These are control pins used in “handshaking” which is establishing a connection.– (This handshaking is built into in the connector making

this a hardware connection.)

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Establishing an connection II

• You may have noticed in another homework that some of the ASCII characters are used in “handshaking” as well. – (software connection)

• Another place we have seen a hint of this connection establishing is in ISDN which had three channels, a D and two B– The D channels request service over the B channels. – The control is separated from the data and is said to be

“out of band.”

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Establishing an connection III• A packet-switched, connection-oriented system like

ATM establishes an SVC, by having the source send out control packets.

• As the control packet is routed to its destination, it establishes the virtual circuit by entering information into the tables of the ATM switches (the “routers” of an ATM network).

• If the destination “accepts” the “request” it returns an accept packet.

• The subsequent data packets are identified not by a destination address but by a virtual channel identifier (VCI).

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VPI/VCI• In place of source and destination addresses,

ATM has VPI (Virtual Path Identifier) and VCI (Virtual Channel Identifier).

• Recall that in broadband connections, the line is broken down into many channels, the VPI determines the route taken (from ATM switch to ATM switch) and the VCI determines the channels used within those connections.

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Comer Fig. 14.2

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Forwarding

• The ATM switches have forwarding tables.

• Instead of looking through a table each time, part of the entry in one ATM switch’s table is the location for the same virtual channel in the next ATM switch’s table.

• This was set up when the virtual circuit was negotiated.

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Comer Fig. 14.3

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Label switching• The forwarding table gives the (physical)

port of the ATM switch through which one should leave and the location of the virtual channel in the next forwarding table.

• In this approach, the data field that directs the packet changes at each ATM switch, this process is known as “label rewriting” or “label switching.”

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UNI-NNI

• The VPI/VCI part changes nature somewhat at the last stage when the cell is about to leave to the network and arrive at a computer. – NNI (network to network interface) between

ATM switches and other ATM switches. – UNI (user to network interface) between

ATM switches and computers.

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The connection-oriented decision

• Setting up a virtual circuit is not worthwhile if one is sending only a few packets.

• It becomes worthwhile when one intends to send a steady (or nearly steady) stream of data between a given source and destination.

• In “live” or real-time audio/video transmission, delay becomes intolerable.

• The choice in ATM cell size had similar motivations.

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Fixed-Length Cell

• Previously we have seen variable payloads – Ethernet up to 1500 bytes– Frame Relay up to 4096 bytes

• ATM fixes the payload size to a relatively small value of 48 bytes– When packets are fixed in size, they are called

“cells”

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The Size Compromise

• 48 bytes was a compromise between Europe who wanted 32 and US/Japan who wanted 64.

• The size is also a compromise between data transmission where large packets are preferred and (live) audio/video where small packets are preferred.

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ATM cell (Fig. 14.1)

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Percentage• An ATM cell has 53 bytes altogether • The first 5 bytes comprise the header

– The VPI/VCI portion is the addressing

• The last 48 bytes comprise the payload• The payload is 90.6% of the cell

• (Payload is the complement of overhead, which in this case is 9.4%.)

48=

48 90.6%

48+5 53

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Compare to Ethernet

• Compare this to the largest Ethernet packet, which has a 1500-byte payload and 26 bytes of header/trailer (including the preamble)

1500=

1500 98.3%

1500+26 1526

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Cell Tax

• The percentage of the cell/packet that is not “actual” data (but protocol data) is significantly higher in ATM compared to Ethernet or Frame relay.

• It has a higher “overhead.”

• This feature is sometimes called the “cell tax.”

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Small but Fast• Having a small, fixed size

– reduces the overhead involved in transmitting cells, this reduces the “latency” — the time delay associated with a switch

• Managing cells is simplified by their fixed size.• Store and forward is faster if there’s less to store and

forward (at a given time).• More done by hardware, less by software, speeds it

up.

– Leads to more predictable transmission rates.

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Hardware based

• The pre-specified bit rates are either 155.520 Mbps or 622.080 Mbps.

• Speeds on ATM networks can reach 10 Gbps.

• Along with Synchronous Optical Network (SONET) and several other technologies, ATM is a key component of broadband ISDN (BISDN).

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jitter• The small size is used to prevent delays in

the transmission of real-time audio/video. • Video and audio require a fairly steady rate. • If a video frame is missing or delayed, the

rate is uneven, and the image is said to “jitter.”

• Long-distance communication involving satellites often has a delay, making communication strained.

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IMA

• Inverse Multiplexing over ATM takes a stream of ATM cells and breaks it up, and transmits it across multiple T1/E1 links, then reconstructs it in the original ATM cell order at the destination.

• Alternative for those who cannot afford a T3 line. • IMA is a User-to-Network Interface standard

approved by the ATM Forum in 1997.

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Video conferencing• A “meeting” between two or more people at

different sites by using computer networks to transmit audio and video data.

• Until the mid 90s, videoconferencing was too expensive for most businesses, but that is changing fast.

• “Many analysts believe that videoconferencing will be one of the fastest-growing segments of the computer industry in the latter half of the decade.”

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The quality of service is not strained

• QoS stands for Quality of Service, which is when a network guarantees a certain throughput.

• That ATM supports a QoS level is a big advantage that it has over Frame Relay and Fast Ethernet.

• ATM providers guarantee their customers a certain amount of bandwidth and a minimal amount of that (end-to-end) latency.

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Best effort

• Transmitting real-time video/audio dependably is difficult in public networks using ordinary "best effort" protocols.

• Using the Internet's Resource Reservation Protocol (RSVP), packets passing through a gateway host can be expedited based on policy and reservation criteria arranged in advance.

• But no guarantees like ATM.

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ATM: Service Type • Constant Bit Rate (CBR): a fixed rate, data is sent

steadily. This is analogous to a leased line. • Variable Bit Rate (VBR): a specified throughput

capacity, but data sent somewhat unevenly. This is a popular choice for videoconferencing (need compression)

• Unspecified Bit Rate (UBR) no guaranteed throughput levels. This is used for applications, such as file transfer, that can tolerate delays.

• Available Bit Rate (ABR) guaranteed minimum capacity but allows data to be bursted at higher capacities when the network is free.

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Compression• The variable bit rate is acceptable if the audio/video

signal is compressed. • Text example: instead of representing all letters by

the same number of bits as in ASCII, one can represent common letters like E as shorter strings and less common letters like Q as longer strings so that on average the number of bits that must be sent is reduced – Lossless: no information lost, just sent or stored more

efficiently – Lossy: some information is lost, but it was considered

superfluous

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References

• http://www.webopedia.com

• http://www.whatis.com

• Computer Dictionary, Mitchell Shnier

• Understanding Data Communications & Networks, Shay